Image sensor arrays typically comprise a linear array of photosensors which raster scan an image-bearing document and convert the microscopic image areas viewed by each photosensor to image signal charges. Following an integration period, the image signal charges are amplified and transferred as an analog video signal to a common output line or bus through successively actuated multiplexing transistors.
For high-performance image sensor arrays, one practical design includes an array of photosensors of a width comparable to the width of a page being scanned, to permit one-to-one imaging generally without the use of reductive optics. In order to provide such a “full-width” array, however, relatively large silicon structures must be used to define the large number of photosensors. One technique to create such a large array is to make the array out of several butted silicon chips. In one design, an array includes 20 silicon chips, butted end-to-end, each chip having 372 active photosensors spaced at 600 photosensors per inch.
In most scanning systems currently in use, the signal coming out of the photosensors during the scanning process is an analog video signal, with the voltage magnitude corresponding to the intensity of light impinging on the photosensor at a given time. The different video levels correspond to the brightness of the reflected area being scanned by a particular photosensor at a particular moment. These analog outputs are digitized and then applied, as needed, to look-up tables, such as to convert the signals to a device-independent color space for further processing.
In a scanner for recording color images, there is typically provided multiple linear arrays of photosensors. Each array includes a translucent filter, causing the particular linear array to be sensitive to substantially one primary color, such as red, blue, and green, to correspond to structures in the human eye. The signals from each filtered linear array are then recorded for assembling a full-color image. There may also be a “white,” or effectively non-filtered, array. With a color input scanner, there is a need for occasional calibration of the linear array outputs relative to each other; to do this, the arrays are typically exposed to white light from a test target of known properties, and their outputs are compared and generally normalized, such as through a look-up table for signal outputs. In a calibration operation, the look-up tables are typically altered depending on current conditions, to result in a normalized output when images are recorded.
U.S. Pat. No. 6,266,438 describes a basic color calibration system for a multi-chip input scanner. U.S. Pat. No. 5,373,374 describes an input scanner in which each of a set of primary-color filters are selectably placed in front of a single-linear-array photosensitive device.